1. Field of the invention
The invention concerns a method of manufacturing a flexible contact lens from a protein biological material in which a hydrogel is made by hydrating the protein polymer, a determined quantity of the hydrogel is enclosed in the cavity of a mold in at least two parts adapted to define, after its parts are locked to each other, a three-dimensional shape with a finished lens surface state, the hydrogel being liquefied at a temperature higher than 50.degree. C., then gelified by cooling in the mold, after which the gel is cross-linked by contact with a cross-linking agent.
2. Description of the prior art
Patent document EP-A No. 0 011 523 describes natural protein polymers of vegetable or animal origin, in particular of the type known as gelatines, having a molecular weight between 10 000 and 100 000 adapted to be swelled by water to form hydrogels with a hydration factor between 0.3 and 10.
Use of these natural protein polymers entails the formation of hydrogels (usually called solutions) containing from 0.5 to 15% by weight of proteins or a mixture of proteins as a result of swelling in water or an aqueous solution, heating the hydrogels to 60.degree..+-.5.degree. C. until they are homogeneous and limpid, and adjusting the pH to between 3.5 and 5.5 approximately. A tanning agent such as ammoniacal iron alum may be added to improve the stability of the hydrogel. Then, while the hydrogel is held at the previous temperature, there is added to it a cross-linking agent such as formaline (37% formaldehyde solution) in a quantity between 0.5 and 15% by weight, the fluid hydrogel is cast in an appropriate mold until the end of cross-linking, and then the lenses removed from the mold are cooled in air at a temperature of not more than 35.degree. C. until the moisture content is below 10%.
If the cast lens is colored, it can be bleached by means of oxidizing agents such as oxygenated water.
Patent document FR-A No. 2 586 703 describes a method of extracting gelified placental collagens also suitable for preparing contact lenses, these collagens being of the type IV enriched kind: their use is very similar to that of the hydrogels as per EP-A No. 0 011 523. However, collagen hydrogels liquefy at temperatures lower than gelatine hydrogels.
It will be noted that an essential benefit of using natural protein hydrogels is their biocompatibility, of decisive importance in tolerance by the wearer: from this point of view human or animal placental collagens show a very remarkable biocompatibility with respect to human and animal organisms, respectively, in the kind in question. Of course, for contact lens applications human placental collagens will preferably be used.
Patent document FR-A No. 2 565 160 describes a method of manufacturing flexible contact lenses from protein polymer of the kind defined in document EP-A No. 0 011 523, in which method a protein polymer hydrogel is placed in a two-part mold at a liquefaction temperature of about 60.degree. to 80.degree. C., at which the hydrogel flows, gelification is brought about by cooling the mold, and the lens is extracted from the mold and placed in contact with a mixture of solvents chosen to avoid degradation of the gelified hydrogel, this mixture containing a cross-linking agent such as an aldehyde; the cross-linking agent diffuses into the gelified hydrogel and renders it insoluble. The lens is then washed and placed in a preserving solution.
It will be understood that the process as per FR-A No. 2 565 160 whereby the lens is shaped simply by gelifying the hydrogel by cooling the mold authorizes a hydrogel "shelf life" (before it is cast) significantly higher than that resulting from the process as per EP-A No. 0 011 523, in which the hydrogel contains the cross-linking agent before it is cast. It should be borne in mind that the "shelf life" of a polymerizable mixture intended for manufacture of articles is the time that elapses between the preparation of the mixture and the time where polymerization is too far advanced for shaping it to be possible.
However, maintaining the hydrogel at the casting temperature for long periods has a number of disadvantages: at the casting temperature the rate of degradation of the protein polymer is not negligible, with modification of the rheological properties of the hydrogel affecting the casting, and a risk of more rapid degradation of the finished contact lenses; also, at the casting temperature there is already significant evaporation of the water constituting the hydrogel, and the viscosity of the hydrogel varies rapidly as a function of the water content; the quality of the casting process depends to a very great degree on the viscosity of the hydrogel. Of course, the casting temperature cannot be reduced in order to reduce the rate at which the hydrogel changes without compromizing the quality of casting, together with the reproducibility of the quantity of hydrogel placed in the molds; on this latter topic, it will be remembered that the quantity of hydrogel placed in a mold is determined by the number of drops deposited and that the mass of a drop depends on the surface tension of the hydrogel, which varies rapidly with temperature.